Memory circuit with switch for selectively connecting an I/O pad directly to a nonvolatile memory cell and method for operating same

ABSTRACT

An integrated circuit operable in a test mode and a normal operating mode, which includes an improved test mode switch. In the test mode of preferred embodiments in which the circuit is an integrated memory chip, the test mode switch is closed so as to connect an input/output (I/O) pad directly with a selected memory cell (so a current/voltage characterization of the cell can be obtained). In the normal operating mode of these embodiments, the test mode switch is open and it isolates the I/O pad from direct connection with the selected cell even under the condition that a transistor of the test mode switch undesirably becomes conductive (e.g., due to low voltage on the I/O pad, inductive coupling, or the like). In preferred embodiments, the test mode switch includes a set of series-connected pass transistors and a isolation voltage switch, the pass transistors pass test signals (indicative of test data to be written to or read from a selected cell) directly between the I/O pad and the selected cell in the test mode, and no signals pass through the pass transistors between the I/O pad and any memory cell in the normal mode. In the normal mode of such preferred embodiments, data to be written from the I/O pad to a selected cell passes through an input buffer before reaching the cell (or data to be read from a selected cell passes through an output buffer before reaching the I/O pad), and the isolation voltage switch holds a channel terminal of at least one of the pass transistors at a fixed supply voltage thereby preventing at least one of the pass transistors from undesirably switching on during the normal mode.

This is a continuation of U.S. Application Ser. No. 08/508,848, filedJul. 28, 1995 (issued as U.S. Pat. No. 5,594,694).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuits (preferably,integrated nonvolatile memory circuits) that are operable either in afirst mode in which input/output (I/O) pads are connected through buffercircuitry to internal circuitry such as an internal logic circuit or anarray of memory cells (so that data can be written to the cells throughan input buffer, or data can be read from the cells through an outputbuffer), or in a test mode in which the I/O pads are connected directlyto the internal circuitry (such as an internal logic circuit or a memorycell array).

2. Description of Related Art

Nonvolatile memory chips (integrated circuits) with higher density arebeing introduced to the market each day. In order to achieve higherdensity, chip manufacturers must continually decrease the size ofelements of the chips (such as the size of each cell of a memory arrayimplemented in each chip). With memory array cells having submicronfeature sizes, the slightest change in processing of one memory cellrelative to another during manufacture results in a big difference inthe behavior of the cells with respect to each other.

Many conventional memory chips operate in either a test mode in whichinput/output ("I/O") pads are connected directly to an array of memorycells, or in a "normal" (or "active") mode in which the I/O pads areconnected through buffer circuitry to the array of memory cells. In thelatter mode (the "normal" mode) the chip can perform read/writeoperations in which data is written to selected ones of the cellsthrough an input buffer (or data is read from selected ones of the cellsthrough an output buffer).

FIG. 1 is a simplified block diagram of a conventional memory chip ofthis type. Integrated circuit 3 of FIG. 1 includes at least one I/O pad30 (for asserting output data to an external device or receiving inputdata from an external device), input/output buffer circuit 10 for I/Opad 30, test mode switch M1, address buffers A0 through Ap for receivingmemory address bits from an external device, row decoder circuit (Xaddress decoder) 12, column multiplexer circuit (Y multiplexer) 14, andmemory array 16 (comprising columns of memory cells, which can benonvolatile memory cells, such as column 16A). Each of address buffersA0 through Ap includes an address bit pad for receiving (from anexternal device) a different one of address bit signals X0 through Xnand Y0 through Ym.

I/O buffer circuit 10 includes a "write" branch and a "read" branch."The write branch comprises input buffer 18. The read branch comprisessense amplifier 19 and output buffer 20. In the normal operating mode ofchip 3 of FIG. 1, chip 3 executes a write operation by receiving data(to be written to memory array 16) from an external device at I/O pad30, buffering the data in the write branch, and then writing the data tothe appropriate memory cell. Also in this normal operating mode, chip 3can be controlled to execute a read operation in which it amplifies andbuffers data (that has been read from array 16) in the read branch, andthen asserts this data to I/O pad 30.

Although only one I/O pad (pad 30) is shown in FIG. 1, typicalimplementations of the FIG. 1 circuit include a plurality of I/O pads,and each I/O pad is buffered by an I/O buffer circuit similar oridentical to circuit 10. For example, one implementation of the FIG. 1circuit includes eight I/O pads, eight buffer circuits identical tocircuit 10, one line connected between the output of the output buffer20 of each buffer circuit and one of the I/O pads (so that eight databits can be read in parallel from buffers 20 to the pads), and one lineconnected between the input of the input buffer 18 of each buffercircuit and one of the I/O pads (so that eight data bits can be writtenin parallel from the pads to buffers 18). Each I/O pad (including I/Opad 30) typically has high impedance when the output buffer is notenabled.

Each of the cells (storage locations) of memory array circuit 16 isindexed by a row index (an "X" index determined by decoder circuit 12)and a column index (a "Y" index output determined by circuit 14). FIG. 2is a simplified schematic diagram of two columns of cells of memoryarray 16 (with one column, e.g., the column on the right, correspondingto column 16A of FIG. 1). The column on the left side of FIG. 2comprises "n" memory cells, each cell implemented by one offloating-gate N-channel transistors N1, N3, . . . , Nn. The drain ofeach of transistors N1-Nn is connected to bitline 13, and the gate ofeach is connected to a different wordline (a different one of wordline 0through wordline n). The column on the right side of FIG. 2 alsocomprises "n" memory cells, each cell implemented by one offloating-gate N-channel transistors N2, N4, . . . , Nm. The drain ofeach of transistors N2-Nm is connected to bitline 15, and the gate ofeach is connected to a different wordline (a different one of wordline 0through wordline n). The source of each of transistors N1, N3, . . . ,Nn, and N2, N4, . . . , Nm is held at a source potential (which isusually ground potential for the chip during a read or programmingoperation).

Each memory cell is a nonvolatile memory cell since each of transistorsN1, N3, . . . , Nn, and N2, N4, . . . , Nm has a floating gate capableof semipermanent charge storage. The current drawn by each cell (i.e.,by each of transistors N1, N3, . . . , Nn, and N2, N4, . . . , Nm)depends on the amount of charge stored on the cell's floating gate.Thus, the charge stored on each floating gate determines a data valuethat is stored "semipermanently" in the corresponding cell. In cases inwhich each of transistors N1, N3 . . . , Nn, N2, N4, . . . , and Nm is aflash memory device (as indicated in FIG. 2 by the symbol employed todenote each of transistors N1, N3, . . . , Nn, N2, N4, . . . , and Nm),the charge stored on the floating gate of each is erasable (and thus thedata value stored by each cell is erasable) by appropriately changingthe voltage applied to the gate and source (in a well known manner).

In response to address bits Y0-Ym, circuit 14 (of FIG. 1) determines acolumn address which selects one of the columns of memory cells of array16 (connecting the bitline of the selected column to Node 1 of FIG. 1),and in response to address bits X0-Xn, circuit 12 (of FIG. 1) determinesa row address which selects one cell in the selected column. Consider anexample in which the column address selects the column on the right sideof FIG. 2 (the column including bitline 15) and the row address selectsthe cell connected along wordline 0 (the cell comprising transistor N2).To read the data value stored in the selected cell, a signal (a currentsignal) indicative of such value is provided from the cell's drain (thedrain of transistor N2, in the example), through bitline 15 and circuit14, to node 1 of FIG. 1. To write a data value to the selected cell, asignal indicative of such value is provided to the cell's gate and drain(the gate and drain of transistor N2, in the example).

In the test mode of the FIG. 1 chip, a selected cell of array 16 isconnected directly to I/O pad 30 (or directly to several I/O padsincluding pad 30 and other I/O pads) so that a current/voltagecharacterization can be performed on the selected cell as follows. Thevoltage at the cell's drain (the potential at which bitline 15 is held,in the example) is controllable since the selected bitline is directlyconnected to an external device through I/O pad 30 (thus, the externaldevice can vary the voltage at which bitline 15 is held, in theexample). By sweeping the voltage on I/O pad 30 during the test mode andmonitoring the resulting current (the current flowing from the selectedcell's drain through bitline 15, circuit 14, Node 1, and switch M1 toI/O pad 30), a current/voltage curve for the cell is obtained.

It should be appreciated that in variations on the FIG. 1 circuit (andin alternative embodiments of the invention to be described below),during the test mode, the voltage at the gate of the selected cell (inaddition to or instead of the voltage at the cell's drain) is controlled(e.g., via direct connection of the gate through access to the wordlinedriver supply through a pad). In the test mode of each such variation(and of each embodiment of the invention), the voltage on the selectedI/O pad is swept (to vary the voltage at the selected cell's drain), andthe current at the selected I/O pad is monitored to obtain acurrent/voltage curve for the cell. Then, voltage of the gate of theselected cell is changed and by sweeping the voltage on the I/O pad andmonitoring the current at the selected I/O pad another current/voltagecurve for the cell is obtained. When this process is repeated for eachof a sequence of gate voltages, a family of current/voltage curves forthe cell is determined.

With reference again to FIG. 1, the function of switch M1 is to switchthe FIG. 1 chip between its test mode and its normal operating mode.Conventionally, switch M1 is an NMOS transistor whose gate receives acontrol signal ("Test Mode Enable") from internal control logic of thechip. The source and drain of M1 are connected in series with I/O pad 30and circuit 14. Switch M1 operates as follows in response to the controlsignal:

M1 is "on" when Test Mode Enable is high (when the value of Test ModeEnable triggers the "test" mode of FIG. 1), and thus M1 functions as apass transistor which passes a signal (a "test" signal) indicative oftest data to be written to or read from a selected cell of array 16(e.g., a current signal indicative of test data read from the selectedcell) between its source and drain (and thus between I/O pad 30 and theselected cell of array 16) without passing such data signal throughbuffer 10; and

M1 is "off" when Test Mode Enable is low (when the value of Test ModeEnable triggers the "normal" operating mode of FIG. 1), so that signals(indicative of data to be written to memory array 16) provided from anexternal device to I/O pad 30 are buffered in input buffer 18 and thenasserted to memory array 16, or signals (indicative of data read frommemory array 16) are asserted from memory array 16 to sense amplifier19, amplified in amplifier circuit 19, and then buffered in outputbuffer 20 and asserted to I/O pad 30. Typically, the "low" value of TestMode Enable is ground potential.

In the normal operating mode of FIG. 1 (with M1 "off"), the FIG. 1circuit executes a write operation as follows. Each of address buffersA0 through An asserts one of bits X0-Xn to decoder circuit 12, and eachof address buffers An+1 through Ap asserts one of bits Y0-Ym to circuit14. In response to these address bits, circuit 14 determines a columnaddress (which selects one of the columns of memory cells of array 16,such as column 16A), and circuit 12 determines a row address (whichselects one cell in the selected column). In response to a write command(supplied from control circuitry not shown in FIG. 1), a signal(indicative of data) present at the output of input buffer 18 (which hasbeen enabled by the appropriate level of the control signal "DATA DRIVERON") is asserted through circuit 14 to the cell of array 16 determinedby the row and column address (e.g., to the drain of such cell). Duringsuch write operation, output buffer 20 is disabled (in response to anappropriate level of control signal OUTPUT ENABLE).

A data latch (not shown) is typically provided between input buffer 18and I/O pad 30 for storing data (to be written to a memory cell)received from I/O pad 30. When the latched data is sent to input buffer18, input buffer 18 produces a voltage at Node 1 which is applied to theselected memory cell. Input buffer 18 is typically implemented as atri-statable driver having an output which can be placed in a highimpedance mode (and thus disabled) during a read operation. Input buffer18 is disabled by asserting (to input buffer 18) an appropriate level ofthe control signal DATA DRIVER ON. In some implementations, thefunctions of the latch and input buffer 18 are combined into a singledevice.

In the normal operating mode (with M1 "off"), the FIG. 1 circuitexecutes a read operation as follows. Each of address buffers A0 throughAn asserts one of bits X0-Xn to address decoder circuit 12, and each ofaddress buffers An+l through Ap asserts one of bits Y0-Ym to circuit 14.In response to these address bits, circuit 14 asserts a column addressto memory array 16 (which selects one of the columns of memory cells,such as column 16A), and circuit 12 asserts a row address to memoryarray 16 (which selects one cell in the selected column). In response toa read command (supplied from control circuitry not shown in FIG. 1), acurrent signal indicative of a data value stored in the cell of array 16(a "data signal") determined by the row and column address is suppliedfrom the drain of the selected cell through the bitline of the selectedcell and then through circuit 14 to sense amplifier 19. This data signalis amplified in amplifier 19, buffered in output buffer 20 (which isenabled by an appropriate level of control signal "OUTPUT ENABLE"), andfinally asserted at I/O pad 30. During such read operation, input buffer18 is disabled (in response to an appropriate level of control signalDATA DRIVER ON).

When reading a selected cell of array 16, if the cell is in an erasedstate, the cell will conduct a first current which is converted to afirst voltage in sense amplifier 19. If the cell is in a programmedstate, it will conduct a second current which is converted to a secondvoltage in sense amplifier 19. Sense amplifier 19 determines the stateof the cell (i.e., whether it is programmed or erased corresponding to abinary value of 0 or 1, respectively) by comparing the voltageindicative of the cell state to a reference voltage. The outcome of thiscomparison is an output which is either high or low (corresponding to adigital value of one or zero) which sense amplifier 19 sends to outputbuffer 20, which in turn asserts a corresponding data signal to I/O pad30 (from which it can be accessed by an external device).

During the test mode, input buffer 18, sense amplifier 19, and outputbuffer 20 are all disabled (in response to appropriate levels of theirrespective control signals DATA DRIVER ON, SENSE AMPLIFIER ENABLE, andOUTPUT ENABLE).

During a write operation in the normal operating mode, control signalSENSE AMPLIFIER ENABLE disables sense amplifier 19. During a readoperation in the normal operating mode, circuit 14 is employed to accessthe desired cell in array 16 and control signal SENSE AMPLIFIER ENABLEenables sense amplifier 19 so that sense amplifier 19 can determine thestate of the selected cell as described.

It is important during a write operation (in the normal operating mode)to provide the wordline of the selected cell with the proper voltage andthe drain of the selected cell with the appropriate voltage level (thevoltage determined by the output of input buffer 18), in order tosuccessfully write data to the cell without damaging the cell.

However, two serious problems often arise during the normal operatingmode of conventional memory chip 3 of FIG. 1. These problems will nextbe described with reference to FIG. 3. When conventional I/O buffer 10executes a "write" operation (during the normal operating mode), it isdesired that conventional switch M1 (an NMOS transistor) is "off" andthus the gate of switch M1 is grounded as shown in FIG. 3 (groundvoltage is the "low" level of Test Mode Enable applied to M1's gate).However, even with the gate of switch M1 grounded, switch M1 mayundesirably turn "on" if the voltage at I/O pad 30 falls sufficientlylow (e.g., to -1 volt, where M1 has a threshold voltage V_(th) equal toabout 0.5 volt) during the normal operating mode (e.g., during a writeoperation in the normal operating mode). If switch M1 undesirably turns"on" at this time, the voltage (at Node 1) applied to the selectedmemory cell is undesirably affected by the voltage at I/O pad 30. Forexample, with switch M1 undesirably "on" during a write operation in thenormal operation mode, even if the output of buffer 18 (which is supposeto have a high value, such as five or six volts, in response to a lowvoltage at I/O pad 30, such as -1 volt), switch M1 (configured as a passtransistor) will undesirably pull Node 1 down and load down the driverof buffer 18. This will change the level of the voltage on the drain ofthe selected memory cell. This effect can cause a failure of theprogramming of the selected memory cell.

A second problem can arise when the circuit of FIG. 1 (and FIG. 3)executes a read operation in the normal operation mode. When aprogrammed bit is read from a selected memory cell, the selected columnof cells (and thus Node 1) will go to a voltage higher than senseamplifier 19's trip point, with the result that the output voltage atI/O pad 30 (the data value read from the selected cell) goes from highto low. If however, switch M1 undesirably turns "on" due to inductivecoupling of I/O pad 30 to a negative value (or the like) during suchread operation, M1 will pull Node 1 down to a voltage lower than senseamplifier 19's trip point, with the result that the output voltage atI/O pad 30 goes from low to high. This effect causes a voltageoscillation at I/O pad 30, which can cause an incorrect data value to beread from the selected memory cell.

The present invention provides a test mode switch that is not subject tothe problems described in the two previous paragraphs.

SUMMARY OF THE INVENTION

The invention is an integrated circuit operable in a test mode and anormal operating mode, which includes an improved test mode switch. Inpreferred embodiments, the integrated circuit is an integrated memorycircuit. In some such preferred embodiments, each memory cell of theintegrated memory circuit is a nonvolatile memory cell such as a flashmemory cell.

In the test mode of an integrated memory circuit embodying theinvention, the test mode switch is in a "closed" (conductive) state inwhich it connects an I/O pad of the chip directly with a selected memorycell (so that a test can be performed to obtain a current/voltagecharacterization of the cell). In the normal operating mode of suchmemory circuit, the test mode switch is in an "open" (non-conductive)state in which it isolates the I/O pad from direct connection with theselected memory cell, even under the condition that a transistor of thetest mode switch undesirably becomes conductive (for example due to lowvoltage on the I/O pad, inductive coupling, or the like). A controlsignal (denoted herein as "Test Mode Enable") applied to the test modeswitch determines whether the test mode switch is in the closed or openstate.

In a class of preferred embodiments, the test mode switch includes a setof series-connected pass transistors and a isolation voltage switch. Inthe test mode, the pass transistors pass test signals directly betweenthe I/O pad and selected internal circuitry of the chip (so that thetest signals are not processed in buffer circuitry). In preferredembodiments in which the chip is a memory chip, the internal circuitryis a selected memory cell and the test signals are indicative of testdata to be written to or read from the memory cell. In the normal mode,no signals pass through the pass transistors between the I/O pad and theselected internal circuitry. Rather, during the normal mode, signals tobe passed from the I/O pad to the selected internal circuitry (e.g.,signals indicative of test data to be written to a memory cell) passthrough an input buffer before reaching the selected internal circuitry,and signals to be passed from the selected internal circuitry to the I/Opad (e.g., data read from a memory cell) pass through an Output bufferbefore reaching the I/O pad. In the normal mode, the isolation voltageswitch holds a channel terminal of at least one of the pass transistorsat a fixed supply voltage, thereby preventing at least one of the passtransistors from undesirably switching on during the normal mode.

In one preferred embodiment, channel terminals of two of the passtransistors are connected together at a Node, and the isolation voltageswitch includes an inverter and an NMOS transistor whose gate receivesan inverted Test Mode Enable signal and whose source and drain areconnected between the fixed supply voltage and the Node. In anotherpreferred embodiment, channel terminals of two of the pass transistorsare connected together at a Node, and the isolation voltage switch is aPMOS transistor whose gate receives the Test Mode Enable signal andwhose source and drain are connected between the fixed supply voltageand the Node.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is block diagram of a conventional memory circuit (implemented asan integrated circuit), including a conventional I/O buffer circuit 10and a conventional test mode switch M1.

FIG. 2 is a simplified schematic diagram of two columns of memory cellsof memory array 16 of FIG. 1.

FIG. 3 is an enlarged diagram of a portion of the circuitry shown inFIG. 1 (in the normal operating mode).

FIG. 4 is a schematic diagram of a variation on FIG. 3 in which a firstembodiment of a test mode switch according to the present invention isprovided as a substitute for conventional test mode switch M1 of FIG. 3.

FIG. 5 is a schematic diagram of a variation on FIG. 3 in which a secondembodiment of a test mode switch according to the present invention isprovided as a substitute for conventional test mode switch M1 of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

We will describe a first embodiment of the invention with reference toFIG. 4. This embodiment is a memory chip (identified by referencenumeral 3') which includes an embodiment of the inventive test modeswitch (circuit 40, which comprises NMOS transistors M2, M3, and M4 andinverter 41) as a substitute for conventional test mode switch M1 ofFIG. 1.

Memory chip 3' of FIG. 4 is identical to conventional memory chip 3 ofFIG. 1 except in that chip 3' includes test mode switch 40 (whichembodies the invention) rather than conventional test mode switch M1.Only test mode switch 40 of chip 3' will be described below, since theforegoing description of those elements of chip 3 other than test modeswitch M1 applies equally well to chip 3', and no purpose would beserved by repeating it below. It is contemplated that in preferredembodiments of the inventive memory chip, each memory cell in eachmemory array of the chip is a nonvolatile memory cell (preferably aflash memory cell).

With reference to FIG. 4, test mode switch 40 has the followingstructure. The input of inverter 41 receives control signal "Test ModeEnable" from internal circuitry of the chip, and the gate of NMOStransistor M3 receives the output of inverter 41 (the inverse of TestMode Enable). The drain of M3 is connected to supply voltage V_(cc), andthe source of M3 is connected to Node 2. NMOS transistors M2 and M4 areboth connected as pass transistors, with the gate of each receivingcontrol signal Test Mode Enable. The source and drain terminals of eachof M2 and M4 will be referred to collectively as "channel" terminals.One channel terminal of M2 is connected to one channel terminal of M4(at Node 2), the other channel terminal of M2 is connected to I/O pad30, and the other channel terminal of M4 is connected to Node 1. Node 1is connected through multiplexer circuit 14 to the selected memory cell.

Test mode switch 40 operates as follows in response to control signalTest Mode Enable:

when Test Mode Enable is high (when the value of Test Mode Enabletriggers the "test" mode of FIG. 4), M2 and M4 are "on" and M3 is "off."Thus, switch 40 is in a "closed" (conductive) state in which M2 and M4function as pass transistors which pass a signal (a "test" signal)indicative of test data to be written to or read from a selected cell ofarray 16 (e.g., a current signal indicative of test data read from theselected cell). The test signal passes through the channel terminals ofM2 and M4 (and thus between I/O pad 30 and the selected cell of array16) without passing through buffer 10; and

when Test Mode Enable is low (when the value of Test Mode Enabletriggers the "normal" operating mode of FIG. 4), M2 and M4 are "off" andM3 is "on." Thus, switch 40 is in an "open" (non-conductive) state inwhich signals (indicative of data to be written to memory array 16)provided from an external device to I/O pad 30 are buffered in inputbuffer 18 and then asserted to memory array 16, or signals (indicativeof data read from memory array 16) are asserted from memory array 16 tosense amplifier 19, amplified in amplifier circuit 19, and then bufferedin output buffer 20 and asserted to I/O pad 30. Typically, the "low"value of Test Mode Enable is ground potential.

When the FIG. 4 circuit executes a write operation during the normaloperating mode of FIG. 4, with Test Mode Enable "low" (grounded), M3"on," and M2 and M4 "off" (i.e., with the gates of M2 and M4 grounded),if the voltage on I/O pad 30 falls sufficiently low (e.g., to -1 volt,where both M2 and M4 have a threshold voltage V_(th) approximately equalto 0.5 volt), M2 can turn "on." This can occur frequently where thememory array is an array of nonvolatile memory cells, since typicalspecifications for nonvolatile memory chips contemplate that I/O padvoltage can fall sufficiently low to turn on a transistor such as M2.

During the normal operating mode of FIG. 4, Node 2 is held at V_(CC)-V_(T) through transistor M3. If I/O pad 30 is brought down to -1 voltexternally, M2 will tend to turn on. Once that happens, both transistorsM3 and M2 are on and there is a current path between V_(CC) and I/O pad30. M3 will try to pull I/O pad 30 up to the point where M2 turns off(e.g., -0.5 volt in this example) and I/O pad 30 will try to pull downNode 2 to -1 volt. The voltage level of Node 2 will be somewhere between(V_(CC) -V_(T)) and -1 volt, at a specific value determined by therelative current driving strengths of M2 and M3. If M3's current drivingcapability is larger than M2's, then Node 2 will end up closer to(V_(CC) -V_(T)) than to -1 volt. If M2's current driving capability islarger than M3's, then Node 2 will end up closer to -1 volt than to(V_(CC) -V_(T)).

The strength of each of the two transistors M2 and M3 is proportional toits width/length ratio (W/L) and the value of (V_(GS) -V_(T))² appliedto it, since these transistors operate in saturation mode. Transistor M2needs to be sized to have sufficient strength for transferring theapplied voltage and current when the test mode enable signal is active.However, during the normal operating mode, if I/O pad 30 goes to -1volt, the (V_(GS) -V_(T)) of M2 is very small and (V_(GS) -V_(T))² iseven smaller. On the other hand, M3 has a "high" voltage (e.g., 5 volts)on its gate, and as Node 2 is pulled down through M2, V_(GS) of M3 willincrease and so will its strength. Although it is conceivable that sizesfor M2 and M3 could be chosen which would cause switch 40 to operatewith low efficiency, one would need to try very hard in order todetermine such sizes. This is true because the only circumstance underwhich switch 40 would not work properly is for Node 2 to be pulled downso low that M4 also turns on (assuming M2 has turned on). Since the gateof M4 is at ground (0 volts), Node 2 would need to come down to a V_(T)below ground (e.g., -0.5 volts in the example) for that to occur. Asdescribed previously, for this to happen, (V_(GS) -V_(T))² of M2 is 0.25and (V_(GS) -V_(T))² of M3 is about 20. So, unless the W/L of M2 is onthe order of 100 times greater than the W/L of M3, this situation wouldnot happen since the current I_(D) of each of M2 and M4 is proportionalto (W/L) (V_(GS) -V_(T))². Therefore, as mentioned previously, it wouldbe necessary to skew the sizes of M2 and M3 radically to design aspecific implementation of switch 40 that would not function properly.

With a properly functioning implementation of switch 40, in the normaloperation mode, even if M2 turns on, it would not be able to pull Node 2down sufficiently to turn M4 on. Thus, Node 1 will remain undisturbed bya voltage at I/O pad 30 which causes M2 to turn on.

In this way, circuit 40 prevents an incorrect data value from beingwritten from input buffer 18 to the selected memory cell even if M2undesirably turns on in response to a low voltage on I/O pad 30 during awrite operation in the normal operating mode.

Circuit 40 also prevents an incorrect data value from being read from aselected memory cell even if M2 undesirably turns on in response to asufficiently low voltage on I/O pad 30 during a read operation in thenormal operating mode. The explanation for this is as follows. Whencircuit 3' reads a programmed bit from a selected memory cell, thebitline of the selected column of cells (and thus Node 1) goes to avoltage higher than sense amplifier 19's trip point, with the resultthat the output voltage at I/O pad 30 (the data value read from theselected cell) goes from high to low. I/O pad 30 might go to a negativevoltage due to inductive coupling. For the reason explained above,switch 40 will not allow Node 2 to go sufficiently low to turn M4 oneven if M2 is turned on. This decouples Node 1 from pad 30, and hencethe noise is not fed back from pad 30 to Node 1 during a read operation.Thus, the output voltage provided from output buffer 20 to I/O pad 30remains unaffected by the transition of M2 from off to on (since M4remains off).

We next describe a second embodiment of the invention with reference toFIG. 5. This embodiment is a memory chip which includes test mode switch50 (comprising NMOS transistors M2 and M4 and PMOS transistor M5) whichembodies the present invention as a substitute for conventional testmode switch M1 of FIG. 3. It is contemplated that in preferredimplementations of the FIG. 5 chip, each cell in the memory array is anonvolatile memory cell (preferably a flash memory cell).

Memory chip 3" of FIG. 5 is identical to conventional memory chip 3 ofFIG. 1 except in that chip 3" includes test mode switch 50 rather thanconventional test mode switch M1. Only test mode switch 50 of chip 3"will be described below, since the foregoing description of thoseelements of chip 3 other than test mode switch M1 applies equally wellto chip 3".

With reference to FIG. 5, test mode switch 50 has the followingstructure. The gate of PMOS transistor M5 receives control signal "TestMode Enable" from internal control logic of the chip. The source of M5is connected to supply voltage V_(CC), and the drain of M5 is connectedto Node 2. NMOS transistors M2 and M4 are both connected as passtransistors, with the gate of each receiving control signal Test ModeEnable. The source and drain terminals of each of M2 and M4 will bereferred to collectively as "channel" terminals. One channel terminal ofM2 is connected to one channel terminal of M4 (at Node 2), the otherchannel terminal of M2 is connected to I/O pad 30, and the other channelterminal of M4 is connected to Node 1. Node 1 is connected throughmultiplexer circuit 14 to the selected memory cell.

Test mode switch 50 operates as follows in response to control signalTest Mode Enable:

when Test Mode Enable is high (when the value of Test Mode Enabletriggers the "test" mode of FIG. 5), M2 and M4 are "on" and M5 is "off."Thus, M2 and M4 function as pass transistors which pass a signal (a"test" signal) indicative of test data to be written to or read from aselected cell of array 16 (e.g., a current signal indicative of testdata read from the selected cell). The test signal passes through thechannel terminals of M2 and M4 (and thus between I/O pad 30 and theselected cell of array 16) without passing through buffer 10; and

when Test Mode Enable is low (when the value of Test Mode Enabletriggers the "normal" operating mode of FIG. 5), M2 and M4 are "off" andM5 is "on." Thus, signals (indicative of data to be written to memoryarray 16) provided from an external device to I/O pad 30 are buffered ininput buffer 18 and then asserted to memory array 16, or signals(indicative of data read from memory array 16) are asserted from memoryarray 16 to sense amplifier 19, amplified in amplifier circuit 19, andthen buffered in output buffer 20 and asserted to I/O pad 30. Typically,the "low" value of Test Mode Enable is ground potential.

When the FIG. 5 circuit executes a write operation during the normaloperating mode of FIG. 5, with Test Mode Enable "low" (grounded), M5"on," and M2 and M4 "off" (i.e., with the gates of M2 and M4 grounded),if the voltage on I/O pad 30 falls sufficiently low (e.g., to -1 volt,where both M2 and M4 have a threshold voltage V_(th) approximately equalto 0.5 volt), M2 can turn "on." However, even if M2 turns "on" underthis condition the voltage at Node 1 will not be affected thereby, aswill be understood by considering the following explanation.

In the described circumstances, Node 2 is held at a voltagesubstantially equal to V_(cc) through transistor M5. If I/O pad 30 isbrought down to -1 volt externally, M2 will tend to turn on. Once thathappens, both transistors M5 and M2 are on and there is a current pathbetween V_(CC) and I/O pad 30. M5 will try to pull I/O pad 30 up to thepoint where M2 turns off (e.g., -0.5 volt in this example) and I/O pad30 will try to pull down Node 2 to -1 volt. The voltage level of Node 2will be somewhere between V_(CC) and -1 volt, at a specific valuedetermined by the relative current driving strengths of M2 and M5. IfM5's current driving capability is larger than M2's, then Node 2 willend up closer to V_(CC) than to -1 volt. If M2's current drivingcapability is larger than M5's, then Node 2 will end up closer to -1volt than to V_(CC).

The strength of each of the two transistors M2 and M5 is proportional toits width/length ratio (W/L) and the value of (V_(GS) -V_(T))² appliedto it. Transistor M2 needs to be sized to have sufficient strength fortransferring the applied voltage and current when the test mode enablesignal is active. However, during the normal operating mode, if I/O pad30 goes to -1 volt, the (V_(GS) -V_(T)) of M2 is very small and (V_(GS)-V_(T))² is even smaller. On the other hand, the gate of M5 is grounded,and as Node 2 is pulled down through M2, V_(GS) of M5 will stay fixed atabout five volts. Although it is conceivable that sizes for M2 and M5could be chosen which would cause switch 50 to operate with lowefficiency, one would need to try very hard in order to determine suchsizes. This is true because the only circumstance under which switch 50would not work properly is for Node 2 to be pulled down so low that M4also turns on (assuming M2 has turned on). Since the gate of M4 is atground (0 volts), Node 2 would need to come down to V_(T) below ground(e.g., -0.5 volts in the example) for that to occur. As describedpreviously, for this to happen, (V_(GS) -V_(T))² of M2 is 0.25 and(V_(GS) -V_(T))² of M5 is about 20. So, unless the W/L of M2 is on theorder of 100 times greater than the W/L of M5, this situation would notoccur since the current I_(D) of each of M2 and M5 is proportional to(W/L)(V_(GS) -V_(T))². Therefore, as mentioned previously, it would benecessary to skew the sizes of M2 and M5 radically in order to design aspecific implementation of switch 50 that would not function properly.

With a properly functioning implementation of switch 50, in the normaloperation mode, even if M2 turns on, it would not be able to pull Node 2down sufficiently to turn M4 on. Thus, Node 1 will remain undisturbed bya voltage at I/O pad 30 which causes M2 to turn on.

In this way, circuit 50 prevents an incorrect data value from beingwritten from input buffer 18 to the selected memory cell even if M2undesirably turns on in response to a low voltage on I/O pad 30 during awrite operation in the normal operating mode.

Circuit 50 also prevents an incorrect data value from being read from aselected memory cell even if M2 undesirably turns on in response to asufficiently low voltage on I/O pad 30 during a read operation in thenormal operating mode. The explanation for this is as follows. Whencircuit 3" reads a programmed bit from a selected memory cell, thebitline of the selected column of cells (and thus Node 1) goes to avoltage higher than sense amplifier 19's trip point, with the resultthat the output voltage at I/O pad 30 (the data value read from theselected cell) goes from high to low. I/O pad 30 might go to a negativevoltage due to inductive coupling. For the reason explained above,switch 50 will not allow Node 2 to go sufficiently low to turn M4 oneven if M2 is turned on. This decouples Node 1 from pad 30, and hencethe noise is not fed back from pad 30 to Node 1 during a read operation.Thus, the output voltage provided from output buffer 20 to I/O pad 30remains unaffected by the transition of M2 from off to on (since M4remains off).

The FIG. 4 embodiment of the invention is preferable to that of FIG. 5for some applications, since PMOS transistor M5 of FIG. 5 may be subjectto a non-negligible latchup effect under some conditions (since M5 willbe formed in a well of N-type semiconductor material in a substrate ofP-type semiconductor material). In contrast, NMOS transistor M3 of FIG.4 will typically not be subject to a latchup problem under the sameconditions.

Preferred embodiments of the invention have been described withreference to FIGS. 4 and 5. Although these embodiments have beendescribed in some detail, it is contemplated that changes from any ofthese embodiments can be made without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. An integrated circuit operable in a selected oneof a test mode and a normal operating mode and including componentcircuits, said integrated circuit including:an input/output padproviding an interface between the integrated circuit and its externalenvironment; a selection circuit configured to connect a selected one ofthe component circuits to a first node; and a test mode switch having aclosed state in the test mode and an open state in the normal operatingmode, wherein the test mode switch connects the input/output paddirectly with the first node in the closed state and isolates theinput/output pad from the first node in the open state, said test modeswitch including:at least one element which is normally nonconductiveduring the open state, but which becomes conductive during said openstate in response to at least one state of the integrated circuit; andan isolation circuit connected to the input/output pad and to the atleast one element, and configured to maintain the input/output pad inisolation from the first node even when said at least one elementbecomes conductive during the open state.
 2. The integrated circuit ofclaim 1 wherein the integrated circuit is an integrated memory circuitincluding:an array of memory cells; and a buffer circuit connectedbetween the input/output pad and the array of memory cells, wherein saidselected one of the component circuits is a selected one of the memorycells, and wherein the buffer circuit includes: a write branch, operablein response to a write condition during the normal operating mode, andconfigured to buffer data received from the input/output pad to generatebuffered input data and to supply the buffered input data to the firstnode for writing to the selected one of the memory cells; and a readbranch, operable in response to a read condition during the normaloperating mode, and configured to buffer a current signal received fromthe first node to generate buffered output data and to supply thebuffered output data to the input/output pad.
 3. The integrated circuitof claim 2, wherein each of the memory cells is a nonvolatile memorycell.
 4. The integrated circuit of claim 1, wherein the test mode switchincludes:a first pass transistor and a second pass transistor connectedin series with the first pass transistors, wherein a channel terminal ofthe first pass transistor is connected to a channel terminal of thesecond pass transistor at a second node; and wherein the isolationcircuit comprises:an isolation voltage switch connected to said secondnode, wherein the isolation voltage switch is configured to hold saidsecond node at a fixed supply voltage during the normal operating mode,thereby preventing one of the first pass transistor and the second passtransistor from switching on even in case the other of said first passtransistor and said second pass transistor switches on during the normaloperating mode.
 5. The integrated circuit of claim 4, wherein theisolation voltage switch is a PMOS transistor having a gate whichreceives a test mode enable signal, a source held at said fixed supplyvoltage, and a drain connected to said second node.
 6. The integratedcircuit of claim 4, wherein the isolation voltage switch includes:aninverter connected to receive a test mode enable signal and configuredto output an inverted test mode enable signal; and an NMOS transistorhaving a gate which receives the inverted test mode enable signal, adrain held at said fixed supply voltage, and a source connected to saidsecond node.
 7. The integrated circuit of claim 4, wherein theintegrated circuit is an integrated memory circuit including an array ofmemory cells, wherein said selected one of the component circuits is aselected one of the memory cells, and wherein the array of memory cellscomprises columns of memory cells, each of said columns having abitline, and the selection circuit includes a multiplexer circuit whichconnects the bitline of the selected one of the memory cells to thefirst node.
 8. The integrated circuit of claim 4, wherein gates of thefirst pass transistor and the second pass transistor are grounded duringthe normal operating mode, a second channel terminal of the first passtransistor is connected directly to the input/output pad, and the fixedsupply voltage is sufficiently high to prevent the second passtransistor from switching on in case the first pass transistor switcheson in response to a voltage below ground voltage on the input/output padduring the normal operating mode.
 9. The integrated circuit of claim 1,wherein the test mode switch includes a set of series-connected passtransistors, and said at least one element is a subset of said set ofseries-connected pass transistors.
 10. An integrated memory circuitoperable in a selected one of a test mode and a normal operating mode,said memory circuit including:an input/output pad providing an interfacebetween the memory circuit and its external environment; an array ofmemory cells; a selection circuit connected to the array and configuredto connect a selected one of the memory cells to a first node; and atest mode switch having a closed state in the test mode and an openstate in the normal operating mode, wherein the test mode switchconnects the input/output pad directly to the first node in the closedstate and isolates the input/output pad from the first node in the openstate, said test mode switch including: a set of series-connected passtransistors, wherein each pass transistor in the set has a gate thatreceives a test mode enable signal, and wherein a channel terminal ofone pass transistor in the set is connected to a channel terminal ofanother pass transistor in the set at a second node; and an isolationcircuit connected to said second node, and configured to hold saidsecond node at a fixed supply voltage during the normal operating mode.11. The memory circuit of claim 10, wherein the array of memory cellscomprises columns of nonvolatile memory cells, each of said columnshaving a bitline, and the selection circuit includes a multiplexerconfigured to connect the bitline of the selected one of the memorycells to the first node.
 12. The memory circuit of claim 11, wherein theisolation circuit includes a transistor having a gate controlled inresponse to said test mode enable signal, a source held at said fixedsupply voltage, and a drain connected to said second node.
 13. Thememory circuit of claim 12, wherein the transistor of the isolationcircuit is a PMOS transistor, and the gate of said PMOS transistor isconnected to receive the test mode enable signal.
 14. The memory circuitof claim 12, wherein the transistor of the isolation circuit is an NMOStransistor, the isolation circuit also includes an inverter connected toreceive said test mode enable signal and configured to output aninverted test mode enable signal, and the gate of said NMOS transistoris connected to receive the inverted test mode enable signal.
 15. Amethod for operating an integrated circuit, said integrated circuitincluding component circuits, an input/output pad providing an interfacebetween the integrated circuit and its external environment, and aselection circuit configured to connect a selected one of the componentcircuits to a first node, said method including the steps of:connectingthe input/output pad directly with the first node in a test mode of theintegrated circuit by placing a switch in a closed state; isolating theinput/output pad from the first node in a normal operating mode of theintegrated circuit by placing the switch in an open state, wherein theswitch has an element that is normally nonconductive during the openstate but which becomes conductive during said open state in response toat least one state of the integrated circuit; and asserting an isolationvoltage to the switch during the normal operating mode to maintain theswitch in the open state during the normal operating mode even when theelement becomes conductive during said open state.
 16. The method ofclaim 15, wherein the switch includes a first pass transistor and asecond pass transistor connected in series with the first passtransistors, a channel terminal of the first pass transistor isconnected to a channel terminal of the second pass transistor at asecond node, and the step of asserting the isolation voltage to theswitch includes the step of:during the normal operating mode, assertinga control voltage to an isolation switch connected to the second node tocause the isolation switch to hold the second node at a fixed supplyvoltage during the normal operating mode thereby preventing one of thefirst pass transistor and the second pass transistor from switching oneven in case the other of said first pass transistor and said secondpass transistor switches on during the normal operating mode, whereinthe isolation voltage is the fixed supply voltage.
 17. The method ofclaim 16, also including the step of:grounding gates of the first passtransistor and the second pass transistor during the normal operatingmode, wherein a second channel terminal of the first pass transistor isconnected directly to the input/output pad, and the fixed supply voltageis sufficiently high to prevent the second pass transistor fromswitching on in case the first pass transistor switches on in responseto a voltage below ground voltage on the input/output pad during thenormal operating mode.
 18. A method for operating an integrated memorycircuit, said memory circuit including an input/output pad providing aninterface between the memory circuit and its external environment, anarray of memory cells, and a selection circuit connected to the arrayand configured to connect a selected one of the memory cells to a firstnode, said method including the steps of:connecting the input/output paddirectly with the first node in a test mode of the memory circuit byplacing a switch in a closed state; isolating the input/output pad fromthe first node in a normal operating mode of the memory circuit byplacing the switch in an open state, wherein the switch has an elementthat is normally nonconductive during the open state but which becomesconductive during said open state in response to at least one state ofthe memory circuit; and asserting an isolation voltage to the switchduring the normal operating mode to maintain the switch in the openstate during the normal operating mode even when the element becomesconductive during said open state.
 19. The method of claim 18, whereinthe memory circuit enters the test mode in response to a test modeenable signal, wherein the switch includes a set of series-connectedpass transistors, each pass transistor in the set has a gate coupled toreceive the test mode enable signal, a channel terminal of one passtransistor in the set is connected to a channel terminal of another passtransistor in the set at a second node, and wherein the method alsoincludes the step of:during the normal operating mode, asserting saidtest mode enable signal to an isolation switch connected to the secondnode to cause the isolation switch to hold the second node at a fixedsupply voltage during the normal operating mode thereby preventing atleast one pass transistor in the set from switching on even in caseanother pass transistor in the set switches on during the normaloperating mode, wherein the isolation voltage is the fixed supplyvoltage.